Debhelper addon to track package versions used to build a package

This script is designed to be run at build-time, and registers in a
file the list of packages declared as build-time dependencies, as
well as build-essential packages, together with their versions, as
installed in the build machine.

This will hopefully help to track packages (auto-)built with package
versions which are known to be buggy, and, more generally, to find
out whether a package needs to be rebuilt because of a significant
change in a package it has a build-time dependency on.